The present invention relates to a manufacturing method of a plasma display panel used for display devices and the like, and especially relates to a manufacturing method of a plasma display panel with an improved dielectric glass layer.
In recent years, there have been high expectations for high-quality large screen televisions such as high-definition televisions (HDTV). As a display panel for such televisions, cathode ray tubes (CRTs), liquid crystal displays (LCDs), and plasma display panels (PDPs) have been used. Although CRTs have superior resolution and picture quality to those of PDPs and LCDs, they are unsuited for large-screens of 40 inches or more in terms of depth and weight. LCDs, meanwhile, have low power consumption and low drive voltages, but their screen size and viewing angle are limited. On the other hand, PDPs enable large screen televisions to be produced, with models in the 40-inch range having been already developed (See xe2x80x9cKino Zairyo (Functional Materials)xe2x80x9d February 1996. Vol.16, No.2, p.7).
FIG. 8 is a perspective view showing a conventional alternate current (AC) type PDP. As shown in FIG. 8, discharge electrodes 72 are formed on the surface of a front glass substrate 71 made of borosilicate sodium by a floating method. A dielectric glass layer 73 is formed so as to cover the discharge electrodes 72, and the surface of the dielectric glass layer 73 is covered with a magnesium oxide (MgO) dielectric protective layer 74. The dielectric glass layer 73, which serves as a condenser, is formed from glass particles having an average particle diameter of 2 to 15 xcexcm.
Address electrodes 76 are formed on the surface of a back glass substrate 75, and a dielectric glass layer 77 is formed so as to cover the address electrodes 76. Barrier ribs 78 and phosphor layers 79 are formed on the surface of the dielectric glass layer 77. Spaces created between the barrier ribs 78 are discharge spaces 80, into which a discharge gas is enclosed.
A high-definition television that is fully compatible with the specification for Japanese xe2x80x9cHiVisionxe2x80x9d broadcasts has achieved 1920xc3x971125 pixels and expectations are growing for the television. The dot pitch of a 42-inch screen has a pixel pitch of 0.15 mmxc3x970.48 mm, and the area of one cell is as small as 0.072 mm2. The area is 7 or 8 times smaller than a 42-inch, high-definition television according to the conventional NTSC (640xc3x97480 pixels, a dot pitch of 0.43 mmxc3x971.29 mm, and a cell area of 0.55 mm2).
As a result, the panel luminance decreases for the high-definition television that is fully compatible with the specification for Japanese xe2x80x9cHiVisionxe2x80x9d broadcasts (See xe2x80x9cDisplay and Imagingxe2x80x9d 1997, Vol. 6, pp. 70).
Furthermore, not only does the distance between discharge electrodes decrease, but also the discharge space becomes smaller. To provide the dielectric glass layers 73 and 77 without degrading their capacities as capacitors in a smaller cell area, it is necessary to make the dielectric glass layers 73 and 77 thinner.
There are mainly three conventional methods for forming dielectric glass layers.
According to the first method, a dielectric glass layer is formed using glass particles that have an average diameter of 2 to 15 xcexcm and the softening point of 550 to 600xc2x0 C. The glass particles are blended with a solvent such as terpineol containing ethyl cellulose or butyl carbitol acetate using three roles, and made into a glass paste. The glass paste is applied to the surface of a front glass substrate by screen-printing. Here, the viscosity of the glass particles has adjusted to 50,000 to 100,000 centipoises beforehand, which is suitable for screen-printing. The glass paste is then dried and fired at around the softening point of the glass particles (550 to 600xc2x0 C.), thereby forming a dielectric glass layer.
In this method, the glass paste is baked at around its softening point. Since the glass does not flow well and is inactive at around the softening point, the melted glass particles hardly react with the electrodes of Ag, ITO, Crxe2x80x94Cuxe2x80x94Cr, or the like. This prevents the resistance of electrodes from rising, or substances constituting the electrodes from dispersing into and staining the glass paste. Also, this method requires only one firing to form a dielectric glass layer. On the other hand, bubbles (pinholes) appear in the dielectric glass layer and lower the withstanding voltage of the layer in this method. Here, the withstanding voltage refers to a maximum voltage applicable to a dielectric glass layer before the glass layer is destroyed and insulating properties begins to degrade.
The second method uses low-melting lead glass particles that have an average diameter of 2 to 15 xcexcm and the softening point of 450 to 500xc2x0 C. (the proportion of PbO is about 75%). After the particles are made into a paste so as to have a viscosity of 35,000 to 50,000 centipoises, the glass paste is applied to a front glass substrate by screen-printing, dried and fired at 550 to 600xc2x0 C., which is about 100xc2x0 C. higher than the softening point of the glass particles. Since the firing temperature is much higher than the softening point, the glass paste flows well. As a result, a dielectric glass layer having a smooth surface (whose surface roughness is about 2 xcexcm) can be obtained. Also, it requires only one firing to form the dielectric glass layer.
However, when the glass paste is activated and flows well, the molten glass particles are likely to react with the compositions of the electrodes such as Ag, ITO and Crxe2x80x94Cuxe2x80x94Cr. As a result, the resistance of electrodes increases, and the dielectric glass layer is stained. Also, large bubbles tend to emerge in the formed dielectric glass layer.
The third method is the combination of the first and the second methods (See Japanese Laid-Open Patent Application No. 7-105855, and No. 9-50769). Which is to say, glass particles having an average diameter of 2 to 15 xcexcm and the softening point at 550xc2x0 C. to 600xc2x0 C. are made into a paste and printed on the front glass substrate by screen-printing. The glass paste is then dried and fired at around its softening point. On the formed dielectric glass layer, another glass paste is applied by screen-printing, which is formed from glass particles whose diameter is in a range of 2 to 15 xcexcm and softening point is at 450 to 500xc2x0 C. The printed glass paste is then dried and fired at 550 to 600xc2x0 C., which is 100xc2x0 C. higher than the softening point, to form another dielectric glass layer.
Such a two-layer construction not only prevents the discharge electrodes from reacting with the glass paste, but also improves the withstanding voltage. However, the manufacturing process of a dielectric glass layer with a two-layer construction is complicated. Furthermore, it is difficult to form a thinner dielectric glass layer, which is necessary to improve panel luminance.
The present invention intends to provide a manufacturing method for a plasma display panel that can overcome the problems associated with the withstanding voltage of a dielectric glass layer.
To do this, the present invention provides a manufacturing method for a plasma display panel by which electrodes are formed on a surface of a substrate in a first process and a dielectric glass layer is formed on the electrodes in a second process, the second process comprising a grinding step of grinding a dielectric glass material; a spheroidizing step of converting each particle of the ground dielectric glass material into a spheroidal form; an applying step of applying a mixture of the spheroidal dielectric glass particles and a binder as a layer to the surface of the substrate on which the electrodes are formed; and a firing step of firing the layer to remove the binder from the layer, thereby forming a dielectric glass layer.
Since a binder used for screen-printing does not easily adhere to the conventional glass particles evenly, a binder applied to some of the glass particles in a large amount do not easily get fired and tend to remain there even after all the glass particles have melted down and a dielectric glass layer is completed. By contrast, a binder can be evenly applied to the surface of glass particles included in a dielectric glass layer, if the particles are converted from irregular-shaped ones into spheroids in the spheroidizing step. In the dielectric glass layer, the binder applied to the glass particles can be fired at almost the same speed and burn out entirely before the burning temperature reaches the softening point of the glass particles. Therefore, gas enclosed in a dielectric glass layer for firing the binder can get away from the layer. In this case, there is a scarce possibility for the gas to stay there in the form of bubbles. This causes an increase in a withstanding voltage of the dielectric glass layer.
In the spheroidizing step, the surface of the ground particles of the dielectric glass material is melted. By doing so, the glass particles are converted into spheroids.
During the melting, the particles of the ground glass material are exposed to a plasma jet. The plasma jet has an effect of melting the surface of the particles and converting them into spheroids.
At the same time, the particles of the ground dielectric glass materials are left in an atmosphere at a temperature of under the softening point of the glass particles. By doing so, the surface of the glass particles melts down and the particles are converted into spheroids.
The glass particles flowing in the plasma jet collide with each other at a high speed and are chipped off and polished, until at last each of the particles is converted into a spheroid.
Note that the second process further comprises a step of classifying the glass particles, which is performed between the spheroidizing step and the applying step, so that a maximum diameter of the spheroidal particles of the dielectric glass material does not exceed a half thickness of the dielectric glass layer after the firing step.
The applying step is performed by placing a dielectric glass sheet on the surface of the substrate, the dielectric glass sheet being obtained by mixing the spheroid glass particles and a thermoplastic resin.
Following those steps, a thinner dielectric glass layer can be achieved.